In many physicochemical systems, both natural and artificial, parameters of interest are measured using a membrane to isolate the process from the measuring system (comprising a probe and analytical instrument). A membrane may also be used to select for the analyte of interest. The analyte must therefore pass through the membrane in a manner which permits quantitative evaluation. The measuring system may take many forms, including mass spectrometry, where the system is referred to as “Membrane Inlet Mass Spectrometry” or MIMS. A MIMS measuring system comprises a membrane inlet probe coupled to a mass spectrometer.
Mass spectrometry is often associated with measurements in the gas phase. It is often necessary to carry out measurements in the liquid phase, as, for example, studying the progress of reactions in biological reactors. Many substances of interest are sparingly volatile, and in any case need to be measured in situ in the liquid phase. Here, membrane technologies allow selective permeation of analytes to the high vacuum system of the mass spectrometer.
The response of a MIMS probe (and therefore the measuring system) for various substances is dependant on many physical factors influencing volatility and diffusivity of the target analyte through the membrane. The presence of external mass transport limitations, set up by hydrodynamic surface (boundary) layers (and potentially including biological films) surrounding the membrane significantly influences the response of the MIMS system. For example, analyte depletion of the region proximate the MIMS probe occurs as analytes diffuse through the membrane inlet of the MIMS probe. This results in reduced response/less accurate readings due to the concentration of analytes in the region proximate the probe not being representative of the concentrations in the bulk of the medium under investigation.